Once I purchase a new phone only, how do I activate it with my carrier?

It depends on which carrier you have service with.  With GSM carriers, such as AT&T, T-Mobile, SunCom, and Cellular One, you only need to remove your SIM card out of the back of the phone and put it in your new phone.  CDMA carriers such as Verizon and Sprint require you to call in the phones serial number to their customer service department.  For more information on GSM and CDMA carriers, see Chapters 5 and 10 of our Buyers Guide.